BANGOR, Maine — Negotiators for Eastern Maine Medical Center and the unionized nurses who work there returned to the bargaining table with a federal mediator and an official state observer on Monday in hopes of hammering out a new three-year contract for the nurses.

Late Monday afternoon, the two sides announced no agreement had been reached, but both expressed general hope that acceptable middle ground may yet be found. Talks will resume Dec. 8.

EMMC nurses staged a one-day strike on Nov. 22, preceded by a two-day lockout. The hospital brought in 215 nurses from outside the area to staff the hospital during the work stoppage. Monday’s talks were the first since the divisive episode.

The major obstacles to agreement include staffing ratios, health insurance and job protection provisions, but the issue of staffing ratios, from the beginning, has been the most visible and contentious. EMMC nurses, backed up by unionized colleagues in other states, maintain that chronic understaffing undermines patient safety and leaves nurses frustrated and overworked.

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The hospital says nurse staffing is in line with national standards, and that patients at EMMC receive high-quality care. And while nurses have said they remain open to negotiation, hospital officials maintain they will not agree to include mandated staffing ratios in the nurses’ contract.

Despite the apparent standoff, “EMMC continues to be hopeful of reaching an agreement,” according to a hospital statement issued Monday afternoon. The statement noted that staffing decisions are influenced by a nurses’ Professional Practice Committee established three years ago in the 2007 contract.

“While we cannot and will not agree to mandated staffing ratios in the [new] contract, EMMC nurses do have input into the quality and safety of the care delivered to our patients, and we’re open to finding new, creative ways to enhance that input,” said employee relations director Paul Bolin in the prepared statement.

Judy Brown, president of the nurses union at EMMC, said the “tone” of the discussions on Monday was improved due to the presence for the first time of a representative from the office of Gov. John Baldacci.

“We are hopeful that we will eventually reach an agreement,” Brown said.

The state has no authority to intervene in the nurses’ labor dispute, but under pressure from the Blaine House to resolve the conflict as soon as possible, the two sides have agreed to allow an impartial observer to sit in on the talks.